Present code division multiple access (CDMA) systems are characterized by simultaneous transmission of different data signals over a common channel by assigning each signal a unique code. This unique code is matched with a code of a selected receiver to determine the proper recipient of a data signal. These different data signals arrive at the receiver via multiple paths due to ground clutter and unpredictable signal reflection. Additive effects of these multiple data signals at the receiver may result in significant fading or variation in received signal strength. In general, this fading due to multiple data paths may be diminished by spreading the transmitted energy over a wide bandwidth. This wide bandwidth results in greatly reduced fading compared to narrow band transmission modes such as frequency division multiple access (FDMA) or time division multiple access (TDMA).
New standards are continually emerging for next generation wideband code division multiple access (WCDMA) communication systems as described in Provisional U.S. patent application Ser. No. 60/082,671, filed Apr. 22, 1998, and incorporated herein by reference. These WCDMA systems are coherent communications systems with pilot symbol assisted channel estimation. These pilot symbols are transmitted as quadrant phase shift keyed (QPSK) known data in predetermined time frames to any receivers within range. The frames propagate in a discontinuous transmission (DTX) mode, so transmission occurs when a user speaks, but no transmission takes place when the user listens. The pilot symbols are equally spaced in time among sixteen predetermined time slots within the frame. Each time slot further includes transmit power control (TPC) and data symbols in both downlink (received) and uplink (transmitted) signals. Received pilot symbols are compared to known pilot symbols to calculate a channel estimate, to demodulate TPC and data symbols and to determine a signal-to-interference ratio (SIR).
A problem arises, however, with increasing velocity of the mobile unit with respect to the base station. Increasingly high Doppler frequencies cause the magnitude and phase of a received signal to change rapidly. Such rapid changes cause greater errors in the channel estimate and result in increased demodulation errors of TPC symbols. These demodulation errors of TPC symbols result in improper transmission power control and compromise communication between a mobile unit and a base station.